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newbie2BS
08-10-2005, 09:49 PM
Newbie needs help.
I have successfully created a fourteen(14) digit display (three lines)·using a BS2 and a MAX7219 driver with 0.3" LEDs (see Nuts and Volts Column #10, "Put your data up in lights using a LED display Chip").
Now I want to supersize·the display with fourteen(14) 4" LEDs. These big LEDs need at least 12 volts at 20 mAmps to operate correctly.
I understand I can use a ULN2803A to power these bigger units (see Stamp Applications no. 6, August 1995). But when I put the ULN2803A in series between the MAX7219 and the LEDs, I get 500 mvDC input (pin 1)·to the ULN2803 but -13 to -7 mvDC on the output side (pin 18) and the LEDs are not on. I measure 12vDC on ULN2803A (pin 10). I know I'm doing something really wacked.
Can anyone give me some advice?

Ryan Clarke
08-10-2005, 09:54 PM
newbie2BS,

Could you post a schematic of your connections, so that we can check your wiring?

Ryan

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Ryan Clarke

Parallax Tech Support
rclarke@parallax.com (mailto:rclarke@parallax.com)

newbie2BS
08-10-2005, 10:07 PM
Hope you can read this schematic.

Chris Savage
08-11-2005, 12:18 AM
newbie2BS,

·· I think you misunderstand the way the ULN2803 works.· The outputs of the MAX7219 need to be high when lighting an anode array on the displays, however, the ULN2803 doesn't do this.· When you make the input of the ULN2803 high, it's output is connected to ground (Vss), which isn't what Common Cathode (CC) displays need on their anodes.

·· By the way, what is the clock source you have listed for?· The clock line on the MAX7219 is normally clocked by a BASIC Stamp pin, but it almost appears as though you have a crystal or something attached?


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Chris Savage
Parallax Tech Support
csavage@parallax.com (mailto:csavage@parallax.com)

newbie2BS
08-11-2005, 04:33 AM
The clock source is a DS1302 with 32.768KHz crystal. It's wired to pins 0,1,2 of the BS2. Works just fine.
So using a ULN2803 is not the way to go. Got any suggestions?

Chris Savage
08-11-2005, 04:38 AM
Okay,

·· I didn't see the DS1302 in your schematic, so I thought that clock value was for the MAX7219.· In any event, the ULN2803 will not work in that fashion.· Do I have any ideas?· Yes, I do...Please see the following app note from Maxim IC.

Using the MAX7219/7221 to Drive Higher Voltage or Current:

http://www.maxim-ic.com/appnotes.cfm/appnote_number/1196



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Chris Savage
Parallax Tech Support
csavage@parallax.com (mailto:csavage@parallax.com)